1. Field of the Invention
This invention relates to a polyacetal resin composition excellent in long-term thermal aging resistance, more particularly to a polyoxymethylene composition which has characteristics of polyoxymethylene such as mechanical properties, moldability, thermal stability and the like and is excellent in long-term thermal aging resistance under high-temperature circumstances.
2. Description of the Related Art
Polyacetal resins are known as engineering resins excellent in mechanical properties, rigidity, creep characteristics, solvent resistance and the like, and used in a wide field including, for example, mechanical moving parts and the like used in automobile, electrical appliances and the like. As the uses of resins are widened and diversified, the demand for quality has become severe. In particular, in the use thereof under high-temperature circumstances, long-term thermal aging resistance is required. However, no conventional polyacetal resins have aging resistance on a satisfactory level, and the use thereof is limited at present.
As a conventional method for preventing the polyacetal resin from being deteriorated under high-temperature circumstances, there has been known a method in which an additive such as a thermal stabilizer, an antioxidant or the like is compounded. For example, JP-B-62-58,387 and U.S. Pat. No. 4,342,680 propose improving the thermal stability with an additive composed of a ternary combination of an amine-substituted triazine, a sterically hindered phenol and a metal-containing compound. JP-A-2-209,944 proposes improving the long-term thermal resistance by the co-use of three kinds of antioxidants with a calcium salt of a fatty acid. Also, JP-B-55-22,508 and U.S. Pat. No. 3,743,614 disclose improving the stability against oxidation and thermal decomposition by use of a hindered phenolic compound together with an alkaline earth metal salt of a carboxylic acid having 10 to 20 carbon atoms and/or an alkaline earth metal hydroxide. Furthermore, JP-B-60-56,748 and UK-A-1,425,771 propose improving the stability against heat and oxygen by use of a hindered phenolic compound and an alkaline earth metal salt of an aliphatic carboxylic acid having 22 to 36 carbon atoms.
Though the long-term thermal aging resistance of a polyacetal resin can be improved by compounding therein an additive, the improvement is still insufficient. The use of such polyacetal resins over a long period of time under high temperature circumstance results in the reduction of mechanical properties and ultimately leads to degradation of the polyacetal resin.
It is therefore an object of the present invention to overcome the disadvantages of conventional polyacetal resins, to provide a polyacetal resin composition excellent in long-term thermal aging resistance, and to provide methods for making the same.
The present inventors have examined the long-term thermal aging resistance of polyacetal resins and have consequently found that a polyacetal resin composition comprising a particular polyacetal, an antioxidant and a basic substance in specific amounts achieves the above-mentioned object, based on which this invention has been completed.
According to this invention, there is provided a polyacetal resin composition which comprises 100 parts by weight of a polyacetal such that the residual fluorine concentration is not more than about 13 ppm and that when the resin is heated at about 230xc2x0 C. for about 30 minutes in nitrogen the concentration of formaldehyde generated is not more than about 500 ppm, from about 0.01 to about 3 parts by weight of an antioxidant and from about 0.001 to about 5 parts by weight of a basic substance.
The polyacetal used in the resin composition of this invention is produced by, for example, the following method:
First of all, the main monomer used in the production of the polyacetal is trioxane which is a cyclic oligomer of formaldehyde and the comonomer used therewith is a cyclic ether represented by the following general formula (I): 
wherein each of R1 to R4 independently represents a hydrogen atom, a C1-C5 alkyl group which is unsubstituted or substituted with 1 to 3 halogen atoms, and each R5 independently represents a methylene or oxymethylene group, which is unsubstituted or substituted with 1 or 2 C1-C5 alkyl groups or 1 or 2 halogen atoms (in this case, p represents an integer of 0 to 3), or each R5 independently represents a divalent group represented by the following formula (II) or (III):
xe2x80x94(CH2)qxe2x80x94Oxe2x80x94CH2xe2x80x94xe2x80x83xe2x80x83(II)
xe2x80x94(OCH2CH2)qxe2x80x94Oxe2x80x94CH2xe2x80x94xe2x80x83xe2x80x83(III)
in which p in the formula (I) is 1 and q in the formula (II) or (III) represents an integer of 1 to 4.
Typical examples of other comonomers include, for example, ethylene oxide, propylene oxide, 1,3-dioxorane, 1,3,5-trioxepane, 1,4-butanediolformal, epichlorohydrin diglycolformal and the like. The concentration of the comonomer is not critical; however, it is usually not less than 0.0005 mole but not more than 0.15 mole per mole of the trioxane.
As the polyacetal in this invention, there can be used either a homopolymer obtained by homopolymerizing the above trioxane or a copolymer obtained by polymerizing a mixture of the above trioxane and the above comonomer.
A suitable polymerization catalyst which can be used in the production of the polyacetal of the present invention includes, but is not limited to, at least one of boron trifluoride, boron trifluoride hydrate, a coordination complex compound of boron trifluoride with an organic compound containing an oxygen atom or a sulfur atom (e.g. an ether such as diethyl ether, di-n-butyl ether, a thioether such as ethylthioether). The polymerization catalyst is preferably a coordination complex compound of boron trifluoride with the above organic compound, and the coordination complex compound includes, but is not limited to, boron trifluoride diethyl ether, boron trifluoride dibutyl ether or mixtures thereof.
During the polyacetal production using the above polymerization catalyst, residual fluorine is accumulated. An important point of this invention is to use a polyacetal having a residual fluorine concentration of not more than about 13 ppm. It is preferable to use a polyacetal having a residual fluorine concentration of not more than about 8 ppm. When the residual fluorine concentration of the polyacetal exceeds about 13 ppm, the use of a polyacetal resin composition under high-temperature circumstances for a long period of time results in reduction of the long-term thermal aging resistance of the resin composition.
As a method for producing a polyacetal having a low residual fluorine concentration, it is effective to control the polymerization catalyst concentration to not more than a certain value during the polymerization. Specifically, it is preferable to control the polymerization catalyst concentration to not more than about 3.0xc3x9710xe2x88x925 mole per mole of trioxane or, if a comonomer is used, per mole of a total of trioxane and the comonomer(s). In particular, in order to obtain a polyacetal having a residual fluorine concentration of not more than about 8 ppm, it is preferable to control the polymerization catalyst concentration to not more than 1.5xc3x9710xe2x88x925 mole per mole of trioxane or, if a comonomer is used, per mole of a total of trioxane and the comonomer(s). When the polymerization catalyst concentration is high during the polymerization and the residual fluorine concentration of the polyacetal produced by the polymerization is more than about 13 ppm, it is possible to wash the polyacetal with a solvent to remove the polymerization catalyst therefrom to reduce the residual fluorine concentration of the polyacetal to not more than about 13 ppm. Specifically, washing methods, include but are not limited to, washing, with hot water, steam or a mixture of water and an organic solvent at a high temperature, a polyacetal obtained by deactivating the polymerization catalyst just after the polymerization or a polyacetal obtained by deactivating the polymerization catalyst and converting the unstable terminal portions present in the polyacetal by decomposition into stable terminals, namely a polyacetal subjected to a so-called terminal stabilization procedure or treatment, or the like. For example, by treating the terminally stabilized polyacetal with an aqueous solution containing about 15% of methanol at a temperature from about 80xc2x0 C. to about 150xc2x0 C. for a period from about 10 min to about several hours (for example, from 1 to 3 hours), the residual fluorine concentration in the polyacetal can be adjusted to about 13 ppm or less. The conditions for this washing treatment are appropriately determined depending upon the residual fluorine concentration of the polyacetal before the washing treatment.
Another important feature of this invention is to provide a polyacetal such that the concentration of formaldehyde generated when the polyacetal is heated at about 230xc2x0 C. for about 30 minutes in nitrogen is not more than about 500 ppm, preferably not more than about 300 ppm. When the concentration of formaldehyde generated under the above-mentioned heating conditions exceeds about 500 ppm, the use of the polyacetal resin composition under high-temperature circumstances results in reduction of the long-term thermal aging resistance thereof.
In the production of the polyacetal such that the concentration of formaldehyde generated is not more than about 500 ppm, the terminal stabilization treatment is accomplished using an extruder by the following three steps: (1) the step of converting the polyacetal to its molten state, (2) the step of adding a liquid mixture of water with an alkaline substance such as triethylamine or the like to the molten polyacetal and kneading the resulting mixture to stabilize the terminals and (3) the step of removing free formaldehyde and the liquid mixture of water with the alkaline substance added in step (2) by degassification under reduced pressure. Also, for further lowering the concentration of any formaldehyde generated, the polyacetal may be subjected several times to the above terminal stabilization procedure.
A resin composition obtained by compounding an antioxidant and a basic substance with the thus obtained polyacetal has excellent long-term thermal aging resistance.
The antioxidant used in this invention is a hindered phenol compound and/or a hindered amine compound.
The hindered phenol compound includes, but is not limited to, n-octadecyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, n-octadecyl 3-(3-methyl-5-t-butyl-4-hydroxyphenyl)propionate, n-tetradecyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, 1,6-hexanediol bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 1,4-butanediol bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], triethylene glycol bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], pentaerythrityl tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 3,9-bis{2-[3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionyloxy]-1,1-dimethylethyl}-2,4,8,10-tetraoxaspiro-(5,5)undecane, N,Nxe2x80x2-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyl]hexamethylenediamine, N,Nxe2x80x2-tetramethylenebis[3-(3-methyl-5-t-butyl-4-hydroxyphenyl)propionyl]diamine, N,Nxe2x80x2-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyl]hydrazine or mixtures thereof.
The hindered amine compound includes, but is not limited to, 4-acetoxy-2,2,6,6-tetramethylpiperidine, 4-stearoyloxy-2,2,6,6-tetramethylpiperidine, 4-acryloyloxy-2,2,6,6-tetramethylpiperidine, 4-(phenylacetoxy)-2,2,6,6-tetramethylpiperidine, 4-benzoyloxy-2,2,6,6-tetramethylpiperidine, 4-methoxy-2,2,6,6-tetramethylpiperidine, 4-stearyloxy-2,2,6,6-tetramethylpiperidine, 4-cyclohexyloxy-2,2,6,6-tetramethylpiperidine, 4-benzyloxy-2,2,6,6-tetramethylpiperidine, 4-phenoxy-2,2,6,6-tetramethylpiperidine, 4-(ethylcarbamoyloxy)-2,2,6,6-tetramethylpiperidine, 4-(cyclohexylcarbamoyloxy)-2,2,6,6-tetramethylpiperidine, 4-(phenylcarbamoyloxy)-2,2,6,6-tetramethylpiperidine, bis(2,2,6,6-tetramethyl-4-piperidyl)carbonate, bis(2,2,6,6-tetramethyl-4-piperidyl)oxalate, bis(2,2,-6,6-tetramethyl-4-piperidyl)malonate, bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate, bis(2,2,6,6-tetramethyl-4-piperidyl)adipate, bis(2,2,6,6-tetramethyl-4-piperidyl)terephthalate, 1,2-bis(2,2,6,6-tetramethyl-4-piperidyl-oxy)ethane, xcex1,xcex1xe2x80x2-bis(2,2,6,6-tetramethyl-4-piperidyloxy)-p-xylene, bis(2,2,6,6-tetramethyl-4-piperidyl)tolylene 2,4-dicarbamate, bis(2,2,6,6-tetramethyl-4-piperidyl)hexamethylene 1,6-dicarbamate, tris(2,2,6,6-tetramethyl-4-piperidyl)benzene 1,3,5-tricarboxylate, tris(2,2,6,6-tetramethyl-4-piperidyl)benzene 1,3,4-tricarboxylate, 1-{2-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxy]-ethyl}-4-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxy]-2,2,6,6-tetramethylpiperidine, a condensation product of 1,2,3,4-butanetetracarboxylic acid with 1,2,2,6,6-pentamethyl-4-piperidinol and xcex2,xcex2,xcex2xe2x80x2,xcex2xe2x80x2-tetramethyl-3,9-(2,4,8,10-tetraoxaspiro-(5,5)undecane)diethanol or mixtures thereof.
In this invention, the antioxidant comprises at least one member selected from the group consisting of the above-mentioned hindered phenol compound and hindered amine compound. The amount of the antioxidant added is 0.01 to 3 parts by weight, preferably 0.01 to 1.0 part by weight, per 100 parts by weight of the polyacetal.
The basic substance used in this invention is a nitrogen-containing compound and/or a metal-containing compound such as a hydroxide, inorganic acid salt or carboxylic acid salt of an alkali metal or alkaline earth metal or mixtures thereof.
The nitrogen-containing compound includes, but is not limited to, dicyandiamide, guanamine (2,4-diamino-sym-triazine), melamine (2,4,6-triamino-sym-triazine), N-butylmelamine, N-phenylmelamine, N,N-diphenylmelamine, N,N-diallylmelamine, N,Nxe2x80x2,Nxe2x80x3-triphenylmelamine, N-methylolmelamine, N,Nxe2x80x2-dimethylolmelamine, N,Nxe2x80x2,Nxe2x80x3-trimethylolmelamine, benzoguanamine (2,4-diamino-6-phenyl-sym-triazine), 2,4-diamino-6-methyl-sym-triazine, 2,4-diamino-6-butyl-sym-triazine, 2,4-diamino-6-benzyloxy-sym-triazine, 2,4-diamino-6-butoxy-sym-triazine, 2,4-diamino-6-cyclohexyl-sym-triazine, 2,4-diamino-6-chloro-sym-triazine, 2,4-diamino-6-mercapto-sym-triazine, 2,4-dihydroxy-6-amino-sym-triazine, 2-hydroxy-4,6-diamino-sym-triazine, N,Nxe2x80x2,Nxe2x80x2,Nxe2x80x2-tetra-cyanoethylbenzoguanamine or mixtures thereof. Preferable are dicyandiamide, guanamine (2,4-diamino-sym-triazine) and melamine (2,4,6-triamino-sym-triazine).
As the other basic substance, there are used metal-containing compounds such as hydroxides, inorganic acid salts, carboxylic acid salts of alkali metals and/or alkaline earth metals. The alkali metals include, but are not limited to, lithium, sodium, potassium or mixtures thereof; and the alkaline earth metals include, but are not limited to, magnesium, calcium, barium or mixtures thereof. The inorganic acid salts of these metals include, but are not limited to, carbonates, silicates, phosphates or mixtures thereof. The carboxylic acid salts of these metals include, but are ot limited to, salts of saturated aliphatic carboxylic acids such as capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, lignoceric acid, cerotic acid, montanic acid, melissic acid, ceroplastic acid or mixtures thereof with these metals and salts of unsaturated aliphatic carboxylic acids such as undecylenic acid, oleic acid, elaidic acid, cetoleic acid, erucic acid, brassidic acid, sorbic acid, linoleic acid, linolenic acid, arachidonic acid, propiolic acid, stearolic acid or mixtures thereof with these metals. Among these carboxylic acid salts, the lauric acid salts, stearic acid salts behenic acid salts or mixtures thereof are particularly preferred.
The basic substance in this invention is at least one compound selected from the group consisting of the above-mentioned nitrogen-containing compounds, the above-mentioned hydroxides, inorganic acid salts, carboxylic acid salts of alkali metals, carboxylic acid salts of alkaline earth metals and mixtures thereof. The amount of the basic substance added is from about 0.001 to about 5 parts by weight, preferably from about 0.005 to about 1.0 part by weight, per 100 parts by weight of the polyacetal.
The time at which the antioxidant and the basic substance are added to the polyacetal may be either before or after the terminal stabilization of the olyacetal; however, this is not critical.
Moreover, the composition of this invention may nclude other additives which have heretofore been known, for example, a thermal stabilizer, a release agent, an antistatic agent, a lubricant, a nucleating agent, a surfactant or the like for imparting the desired characteristics to the composition depending upon the purpose, and in addition thereto, there can be added an organic macromolecular or polymer material, an inorganic or organic fibrous, powdery or plate-like filler, a pigment or the like or mixtures thereof.